The biomedical industry seeks to replace stainless steel hypodermic injection needles with needles that have small diameters, sharper tips, and which can provide additional functionality. The advantages of smaller diameters and sharper tips are to minimize pain and tissue damage. Desirable additional functionality for a hypodermic injection needle includes the capability of providing integrated electronics for chemical concentration monitoring, cell stimulation, and the control of fluid flow, such as through an integrated valve or pump.
Integrated circuit technology and single crystal silicon wafers have been used to produce hypodermic injection needles. A "microhypodermic" injection needle or "microneedle" is described in Lin, et al., "Silicon Processed Microneedle", Digest of Transducers '93, International Conference on Solid-State Sensors and Actuators, pp. 237-240, June 1993. Another microneedle is described in Chen and Wise, "A Multichannel Neural Probe for Selective Chemical Delivery at the Cellular Level," Technical Digest of the Solid-State Sensor and Actuator Workshop, Hilton head Island, S.C., pp. 256-259, Jun. 13-16, 1994. The needles described in these references have common elements since they are both based on the process flow for a multielectrode probe. In particular, both processes rely on heavily boron doped regions to define the shape of the needle and the utilization of ethylenediamine pyrocatechol as an anisotropic etchant.
Lin, et al. describe a fluid passage that is surface micromachined and utilizes a timed etch to thin the wafer such that an approximately 50 .mu.m thick strengthening rib of single crystal silicon remains. In contrast, Chen and Wise bulk micromachine a channel into the microneedle using an anisotropic etch and all of the single crystal silicon comprising the shaft of the needle is heavily boron doped so the timing of the anisotropic etch to form the shape of the needle is less critical.
There are a number of disadvantages associated with these prior art devices. The single crystal silicon strengthening rib in the Lin, et al. microneedle is naturally rough and is difficult to reproduce due to the tight tolerance on the timed etch. The Chen and Wise microneedle results in walls approximately 10 .mu.m or less in thickness and the shape of the fluid channel defines the shape of the silicon comprising the structural portion of the needle. Therefore, small channels lead to thin needles and large channels lead to large needles. This is a problem when a needle with a small channel but large needle cross-section is desired. Often, large needle cross-sections are necessary, such as those 50 .mu.m thick or greater, to obtain a stronger microneedle, but since the fluid flow rate is dependent on the cross-section of the needle, a large needle may not provide the necessary flow resistance.
The Lin, et al. and Chen and Wise microneedles share the drawback that they rely on the use of boron doping to define the shape of the needle. This requires a long (approximately 17 hour or greater), high temperature (approximately 1125.degree. C.) step which is expensive. In addition, the chosen anisotropic etchant is ethylenediamine pyrocatechol, which is a strong carcinogen, making production dangerous and therefore leading to further expenses. Finally, since both of these microneedles utilize an anisotropic etchant to produce the shape of the microneedle, limitations are placed on the geometry of the needle. For the needle to be "sharpest", it is preferred for the tip of the needle to originate from a near infinitesimally small point and taper continuously, without step transitions, to the full width of the shaft of the needle. Such a geometry is not possible using the techniques described in Lin, et al. and Chen and Wise. In particular, the needles produced using those techniques have abrupt step transitions, largely attributable to the use of the anisotropic etchant.
Thus, it would be highly desirable to provide improved microneedles and processes of fabricating microneedles to overcome the shortcomings associated with prior art devices.